DSA interview question

Intersection Point of Two Linked Lists (Y-Shaped)

Medium · Data structures · Topics: linkedlist, two pointer

The problem

Two delivery routes are stored as singly linked lists of stops, and at some stop they merge into the same physical chain of nodes, forming a Y shape. The routing system needs the first shared node (the node itself, not just an equal value), or nullptr if the routes never meet. A hash set of one list's nodes works in O(m) memory. The elegant O(1)-memory trick walks two pointers and, when one reaches the end, restarts it at the other list's head. After at most one switch each, both have walked the same total distance and arrive at the intersection together.

Example: A = 4 -> 1 -> 8 -> 4 -> 5, B = 5 -> 6 -> 1 -> 8 -> 4 -> 5, sharing 8 -> 4 -> 5 → the node with value 8.

Function signature

Node* getIntersectionNode(Node* headA, Node* headB);  // first shared node, or nullptr

Test cases

Input (A-only part, B-only part, shared part)	Output	Explanation
`([4, 1], [5, 6, 1], [8, 4, 5])`	node 8	The equal value 1 is not shared: only node identity counts
`([1, 9, 1], [3], [2, 4])`	node 2
`([2, 6, 4], [1, 5], [])`	nullptr	No shared part
`([], [], [7])`	node 7	Both lists are the same list
`([1, 2, 3], [], [4])`	node 4	B starts at the shared part

(Confirm: the lists are acyclic and unmodified; intersection means the same node in memory, not equal values.)

How interviewers phrase it

"Find the node at which two singly linked lists intersect" (LeetCode 160).
"Can you do it in O(m + n) time and O(1) memory without modifying the lists?"

Clarifying questions to ask

Does intersection mean the same node, or equal values? (Same node.)
Can the lists have cycles? (Assume not.)
Can I modify the lists?

Follow-ups to expect

"Why does switching heads work?"
"What's the length-difference approach?"
"What's the hash-set approach?"
"What if there's no intersection?"
"Complexity?"

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